1. Field of the Invention
The present invention relates to a method for direct cooling of a hot-rolled wire rod.
2. Description of the Prior Art
At present, as a controlled cooling method of a hot-rolled wire rod, the Stelmor method is a typical method which is now widely used. In this Stelmor method, a wire rod having been hot-rolled at a temperature of 850.degree. C. to 900.degree. C. is firstly coiled into a form of series of loops by a coiler, and the wire rod is dropped and introduced to a conveyer and is transported thereon in a form of series of loops. And then, the wire rod is forced to rapidly be cooled by an air-blast at a rate of 10 m to 50 m/sec. from the back side of the conveyor during the transportation, thereby to strengthen the wire rod.
The capability of the cooling depending on such air blast cooling, however, is limited, of itself, to a certain extent. When it comes to a wire rod, for example, of 11 mm in diameter, the speed of this air blast cooling becomes so low as to be approximately at a rate of 5.degree. to 10.degree. C./sec. When a wire rod of high carbon steel is produced by this air blast cooling, because of the low speed of the air blast cooling, the wire rod is reduced to being low in strength, as well as ductility, compared with that which is produced in off line lead patenting. Furthermore, when a wire rod of low or medium carbon steel with a so called supercooling structure such as bainite or martensite is to be produced, it is indispensable to add to the steel elements such as Mn, Cr and Mo for improving hardenability. This addition is also disadvantageous in increasing production cost. In the case of direct hardening of stainless steel, a wire rod with a mild property cannot be produced because, due to its slow speed of cooling, carbides are precipitated during the cooling process.
As prior art means to cover this disadvantage, various methods, for example, a method have been proposed of using a warm water or salt bath as the direct patenting method, or a method of putting a hot-rolled wire rod into a water bath as a direct quenching. But, by means of the warm water, a speed of this water cooling cannot match that of the lead patenting and by the salt bath, the dissolving of the salt requires such a time that the running cost is increased. As to the water bath method, it cannot be employed for multi-purpose use.
Furthermore, various methods of increasing the cooling capability of the Stelmor method have been disclosed in Japanese Patent publications. Namely, (1) in Japanese Patent Application Laid Open (KOKAI) No. 112721/76, water of 0.01 to 0.05 l/air blast of 1.0 m.sup.3 is used for a spray; (2) in Japanese Patent Application Laid Open (KOKAI) No. 138917/78, an air blast which is mixed with water of 0.06 to 0.27 l/Nm.sup.3 into mist is used; (3) in Japanese Patent Application Laid Open No. 214133/87 (KOKAI), moisture is blown away by means of hot air after a wire rod is rapidly cooled by using spray water; and (4) in Japanese Patent Application Laid Open No. 31831/84 (KOKAI), groups of water cooling nozzles are placed above conveyer rollers, the upper surface of an air-cooling chamber is sloped along the direction of conveying the wire rod, and water cooling is carried out, the water being discharged away into both sides of the conveying direction. Furthermore, some concepts of methods and apparatuses for the cooling in said Japanese Patent Application Laid Open Nos. 214133/87 and 31831/84 are suggested.
These prior art methods, however, are disadvantageous in several points. The prior arts mentioned in above (1) and (2) describe a method wherein a wire rod having its loops overlapped is simply applied to rapid cooling, which does not solve the problem of keeping the cooling speed constant and uniformly cooling the wire rod. The prior art described in above (3) is that a wire rod with its loops overlapped are only rapidly cooled from the above. Therefore, this art also fails to solve the aforesaid problem. In addition, this art blows away drops of water on the wire rod after the rapid cooling. But, in the case of a supercooling wherein such drops of water which are required to be blown away remain on a wire rod after cooling, structures of bainite or martensite are inevitably formed. As a result of this, the ductility of the wire rod becomes poor. Furthermore, in the prior art method cited in (4), the cooling is carried out exclusively by means of water cooling from above and the water is discharged to an off line, and therefore, the cooling from below makes no difference from that done conventionally. The concepts of the methods and the apparatuses mentioned above in respect of the Patent Application Laid Open Nos. 214133/87 and 31831/84 do not show specific ideas and therefore, fail to teach how to obtain uniform cooling speed.